Thanks for the comprehensive review which was very interesting. You may be interested to know that Andreas Spiess has hacked the Tradfri. See his videos #140, #144 ‘ where he controls them with MQTT and also CoAP. However I will be interested to see your future MQTT implementation.
I also have an NSPanel Pro (which also has a zigbee gateway) and Tradfri and a Moes zigbee works with it. I assume they use the same firmware on the gateway as well.